fan speeds up and down???? WTF

My friend has the emachine (really, it's my friend) who's fan can be heard speeding up and down alot. I mean ALOT! It seems to do it when there's a processing load.
The machine is a t35XX and its about 3 months old.
This is new to the machine.
Any ideas as to why it does this?

Hey it's really common to have the fan speeding up and down.
Newer processor have something like dynamic switching fan control so that fan speeds only when needed. I'd like to share my experience once again, i have a pentium 4 which does not support this technology, and annoyingly 10 minutes after boot it'll go to 5300RPM! i'm sure you;re more fortunate than me at that time. p.s. you could ask everybody how noisy is 5300RPM (sometimes it even to 5500!)

A greater processing load means more heat output, which leads to higher fan RPMs. It's a natural thing and the only thing you can do is buy headphones and turn up the volume to drown out the noise or buy speakers to do the same!

My friends eMachine does this. It ussually does this when it is handling an enormous task. The funny thing is sometimes the fans spinning at 10,000RPM and after it boots is goes 18RPM... Seriously. If I remeber correctly bios updates can fix fan problems. Look into that.